Several reasons.The ability to elevate bipedally helps increase visible range significantlyWalking/running is MUCH more efficient as weight is borne by long-bones of the legs rather than directly on energy-consuming muscles. Also, vertical posture minimizes body surface-area exposed to the sun (it's above us) which keeps body temperature manageable.

cefm:Several reasons.The ability to elevate bipedally helps increase visible range significantlyWalking/running is MUCH more efficient as weight is borne by long-bones of the legs rather than directly on energy-consuming muscles. Also, vertical posture minimizes body surface-area exposed to the sun (it's above us) which keeps body temperature manageable.

while those are great benefits, the biggest is the synergy of the visible range combined with the ability to utilize the opposable thumb in conjunction with a higher-level brain in a multi-tasking effort (spear/club, carrying food or other resource, etc).

Leader O'Cola:cefm: Several reasons.The ability to elevate bipedally helps increase visible range significantlyWalking/running is MUCH more efficient as weight is borne by long-bones of the legs rather than directly on energy-consuming muscles. Also, vertical posture minimizes body surface-area exposed to the sun (it's above us) which keeps body temperature manageable.

while those are great benefits, the biggest is the synergy of the visible range combined with the ability to utilize the opposable thumb in conjunction with a higher-level brain in a multi-tasking effort (spear/club, carrying food or other resource, etc).

The real reason is that if we had four legs and two arms women would need disposable vaginas to give birth to that.